Scan detector outputs are used to synchronize video data and control the video transfer rate in raster output scan (ROS) systems. The video data rate is adjusted for each scan line based on the time between scan detector outputs of the previous scan line. Raster output scan (ROS) printer quality is directly affected by scan detector malfunction. The scan detector input is a laser scanning beam and a pulse is produced as the beam passes over the center of a photodiode. That is, the scan detectors in a raster output scan printer develop pulses used to synchronize and control the transfer rate of each line of video data. A laser scan beam passes over a photodiode and causes a scan detector output pulse. The pulse circuit is enabled when the photodiode signal exceeds a threshold value. This threshold is set above the maximum expected noise level and below the minimum expected signal level. Normal expected signal levels increase when the printer photoreceptor ages and the laser intensity is adjusted to compensate. Prior art practice provides low safety margins on the scan detector function and can be the first circuit to fail in an operating output printer.
The present invention concerns a technique of improving scan detector margins by an order of magnitude so that the scan detector threshold margins are improved to a point where the scan detector function is the last function to fail as the ROS system degrades.
Previous techniques have utilized a detector threshold value which is a compromise setting between the largest expected noise with the brightest laser and the smallest expected signal with the dimmest laser. The present technique continuously monitors beam intensity/signal and maintains the threshold at a selected percent of the signal value. The disclosed scan detector includes a peak detector function and the output of this function provides the input to a variable threshold circuit.
According to the present invention, feedback from a peak detector circuit adjusts the threshold voltage to approximately the selected percent of the peak voltage. When the signal voltage exceeds the threshold voltage, the crossover circuit is enabled and a scan pulse is generated when crossover is detected. The scan detector ignores noise signals below the threshold voltage.